Experiments are in progress to determine the gene(s) responsible for certain biological characteristics of influenza viruses. For example, the genes which are associated with high yield characteristics of influenza viruses have been identified. Other experiments have led to the identification of the gene which confers amantadine sensitivity (resistance) to different influenza viruses. Another line of experiments is aimed at a characterization of the genome of influenza B viruses. Studies are underway to establish a complete genetic map of influenza B viruses and to analyze genetic drift among these viruses.